The present invention relates to a method and an apparatus for preventing external detection of signal information in video signals occurring in, and being emitted from, a display unit, or a similar unit, and comprising substantially consecutive frame or field signals, each consisting of substantially consecutive line signals.
Display units are widely used as components in, for example, data processing systems in which confidential information is processed and stored, and also in similar units, such as matrix printers. The video signals occurring in these units and containing components of radio frequency, are emitted therefrom and can be intercepted distantly from the display unit itself. By such detection, the information can comparatively easily be displayed on another display unit. Since it is important, both from commercial and legal standpoints, that such information does not leak, efforts have been made to either reduce the emission or alter it in such a manner that detection of the information is rendered more difficult or even impossible.
It is known to reduce the emission of video signals by encapsulating or shielding the equipment at issue, such that the signal strength of the emitted signals becomes too low to be detected. This, however, is a very expensive solution.
It is also known to emit an interfering signal in the form of white noise around the equipment, which renders the interception more difficult, but it is nevertheless possible to detect the video signal information by using advances computer analysis. Furthermore, this technique may create more problems than it solves, since the requires noise signals may exceed the standards of radio interference other adjacent equipment is knocked out.
It is, moreover, known to emit, instead of white noise, an interfering signal with properties similar to those of the video signals. Thus, existing radio interference standards can be maintained, while the detection of video signal information is rendered very difficult.
In the methods of the type mentioned by way of the introduction, it is thus previously known to emit a phantom signal in the form of at least one pseudo-random bit signal sequence with properties similar to those of the video signal.
Apparatuses for carrying out these methods comprise a generator for generating the phantom signal, and an aerial for emitting the phantom signal from the generator.